It is well known that wind power has gained great relevance in recent years in Spain, Europe and the rest of the world. All forecasts point to a sustained growth in wind power generation worldwide. Energy policies of the most advanced and richest countries include among their goals an increased presence of wind power.
Within this context, offshore wind farms are beginning to appear, confirming the expectation of great growth in the use of this technology in coming years. Offshore wind farms clearly entail greater costs, depending of course on the depth of the water at their location, but the wind quality is better, wind speeds are higher and turbulence is lower, resulting in more production hours which, in addition to the higher density of air at sea level generates higher income than land-based wind farms, compensating for the higher initial investment costs. In fact, it is now common, particularly in Germany, Great Britain and Scandinavian countries to promote and build offshore wind farms, with a great number of such farms being studied, in line with the expected growth of this type of wind farms, closely linked to strategic goals set by governments for reaching specific renewable energy production quotas. The trend towards using turbines with greater power and size in order to reduce the unit costs of the installed power has been constant in the development of wind turbines, particularly so for offshore wind power. Nearly all large wind turbine manufacturers are studying or in the later stages of developing high power models, with 3 or more megawatts, adapted to marine conditions, which are particularly demanding.
This power escalation and the particularly demanding marine conditions in turn imply a considerable increase in the demands on the substructure that must support the turbines, which requires developing novel concepts for said substructure with increased capacity, optimum strength and a competitive cost, particularly if the substructure will be used in locations with great depth, which may be advisable in some circumstances. Floating solutions have been proposed for these sites, all of which have been built so far have used a metal substructure.
Among the main drawbacks and limitations of known floating solutions are the following:                The installation of substructures implies high costs related to the scarce and costly marine transportation, handling and lifting of the foundation, shaft and turbine elements.        Steel has a limited duration in the marine medium due to the aggressive conditions of humidity and salinity, particularly in tidal movement areas. Consequently, maintenance requirements are high and costly. Together with the high sensitivity of metal structures to fatigue loads, this means that the useful lifetime of the metal components of the substructure is limited.        Steel substructures are highly sensitive to collisions from ships, icebergs and drifting objects in general.        There are uncertainties resulting from the variability in the cost of steel, considerably greater than that for concrete.        Certain existing solutions present a limited stiffness for the substructure shaft, which limits the capacity for greater heights of the substructure and size of the turbines, particularly with foundation solutions with a limited stiffness, with is the most common situation in off-shore installations.        Great dependency on specific marine lifting and transportation, which are in limited supply.        
With regard to the manufacturing material, structural concrete turns out to be an optimum material for systems on water, particularly marine offshore systems. In fact, although the use of metal structures is predominant in mobile floating elements, as an extension of naval practice and always linked to continuous maintenance, concrete is instead an advantageous alternative and is therefore more common in all types of fixed maritime systems (ports, docks, breakwaters, platforms, lighthouses, etc.). This is mainly due to the durability, robustness and structural strength, reduced sensitivity to marine corrosion and practically maintenance-free service of structural concrete. With a proper design, fatigue sensitivity is also very low. Its useful lifetime generally exceeds 50 years.
Moreover, concrete is advantageous due to its tolerance in case of impact or collisions, and can be designed for example to withstand forces generated by drifting ice or the impact from small ships, as well as due to the simplicity and economy of any necessary repairs.
Structural concrete is also a universal system material, and the raw material and system materials are accessible worldwide and have moderate costs.
For this reason, concrete is increasingly used to build offshore substructures, although until now it has been generally used for substructures with foundations on the seabed, and therefore for small depths or complex structures.